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POArithmeticException thrown during BigDecimal.divide
 

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    <p>I thought <a href="http://java.sun.com/javase/6/docs/api/java/math/BigDecimal.html" rel="noreferrer"><code>java.math.BigDecimal</code></a> is supposed to be The Answer&trade; to the need of performing infinite precision arithmetic with decimal numbers.</p> <p>Consider the following snippet:</p> <pre><code>import java.math.BigDecimal; //... final BigDecimal one = BigDecimal.ONE; final BigDecimal three = BigDecimal.valueOf(3); final BigDecimal third = one.divide(three); assert third.multiply(three).equals(one); // this should pass, right? </code></pre> <p>I expect the <code>assert</code> to pass, but in fact the execution doesn't even get there: <code>one.divide(three)</code> causes <code>ArithmeticException</code> to be thrown!</p> <pre><code>Exception in thread "main" java.lang.ArithmeticException: Non-terminating decimal expansion; no exact representable decimal result. at java.math.BigDecimal.divide </code></pre> <p>It turns out that this behavior is explicitly documented in the <a href="http://java.sun.com/javase/6/docs/api/java/math/BigDecimal.html" rel="noreferrer">API</a>:</p> <blockquote> <p>In the case of <code>divide</code>, the exact quotient could have an infinitely long decimal expansion; for example, 1 divided by 3. If the quotient has a non-terminating decimal expansion and the operation is specified to return an exact result, an <code>ArithmeticException</code> is thrown. Otherwise, the exact result of the division is returned, as done for other operations.</p> </blockquote> <p>Browsing around the API further, one finds that in fact there are various overloads of <code>divide</code> that performs <em>inexact</em> division, i.e.: </p> <pre><code>final BigDecimal third = one.divide(three, 33, RoundingMode.DOWN); System.out.println(three.multiply(third)); // prints "0.999999999999999999999999999999999" </code></pre> <p>Of course, the obvious question now is "What's the point???". I thought <code>BigDecimal</code> is the solution when we need <em>exact</em> arithmetic, e.g. for financial calculations. If we can't even <code>divide</code> exactly, then how useful can this be? Does it actually serve a general purpose, or is it only useful in a very niche application where you fortunately just don't need to <code>divide</code> at all?</p> <p>If this is not the right answer, what <em>CAN</em> we use for exact division in financial calculation? (I mean, I don't have a finance major, but they still use division, right???).</p>
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    1. COBy the way, I'd really appreciate insight from people who've worked with financial software. I often heard `BigDecimal` being advocated over `double`, and I'd like to hear how they deal with `divide`.
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    2. COI'm working on a very complex Java apps displaying lots of financial information in JTables (btw, JTables so complex and so often updated that I was forced to use several of the http://java.sun.com/products/jfc/tsc/articles/ChristmasTree/ optimizations to have my JTables performs acceptably on fast machines). **The** two things that stand out: the unit is typically the cent and I've got my own NumDiv rational abstraction. Note that it is not a shortcoming of the Java platform: at worst it's a shortcoming, if any, of the default JAVA APIs.
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